Impedance pneumography usually is used in monitoring the respiratory activity of spontaneously breathing patients and employs for signal detection the electrical impedance of the thorax, which impedance is influenced by the patient's respiratory activity and thus changes as a function of time.
It is generally known that an impedance pneumography signal does not only have a respiration signal component, but has additional artifacts superimposed thereon, including a cardiac artifact signal component which indicates the impedance change occurring during the blood flow associated with systole. Additional parameters disturbing the impedance pneumography signal are, for example, spontaneous motion artifacts of the patient. While motion artifacts are difficult to eliminate, there are a number of methods of filtering out the cardiac artifact signal component from an impedance pneumography signal in order to obtain a respiration signal.
For example, European Patent EP-B1-0082655 and related U.S. Pat. Nos. 4,537,196 and 4,582,068 disclose a method of processing a physiological signal (which signal may be an impedance pneumography signal) wherein the cardiac artifact signal masking the respiration signal is assumed to be periodic so that, for calculation of a filtered signal, the artifact frequency, which is assumed to be constant, is ascertained at first and thereafter a signal of the particular frequency in the impedance pneumography signal is suppressed in a digital filter to produce the filtered signal. This known method requires a threshold detection circuit for determining a threshold value as a function of preceding levels of previously filtered impedance pneumography signals. The threshold detection circuit is fed with the filtered impedance pneumography signal in order to produce a periodic signal indicating recurrence of the respiration signal when the filtered functional signal exceeds the threshold value of the threshold detection circuit. This known filtering technique is capable of suppressing only the basic component of a cardiovascular artifact occurring with a fixed periodicity. However, the requirement of maintaining periodicity of the disturbing signal is not met in most practical situations; therefore, the results obtained by this known method are unsatisfactory.
EP-A2-0048591 discloses a further filtering technique for obtaining a respiration signal from an impedance pneumography signal by filtering out a cardiac masking signal. According to this known method the heart rate is measured by means of an electrocardiograph to determine the heartbeat period which is used for delaying the impedance signal on the input side by one heartbeat period and for subtracting this signal thereafter from the impedance signal. This known filtering technique is also inadequate when the cardiovascular artifact is not sufficiently periodic.
W089/01312 discloses a method of processing impedance signals which includes filtering out a respiration signal component from the signal representing the impedance of the thorax. For suppressing the stronger impedance change of the thorax due to respiration as compared to the impedance change due to heart activity, a clamping circuit is activated in synchronization with the heart activity at a moment of time before the beginning of the mechanical systole. This circuit is only opened during the duration of the mechanical systole so that the output signal represents only the voltage fluctuation due to the mechanical systole. However, in nature the cardiovascular signal is masked by a slight respiration signal component during this period of time as well, thus making this filtering technique inaccurate.
U.S. Pat. No. 3,976,052 reveals a method of deriving a respiration signal from an impedance pneumography signal by filtering out a cardiac artifact signal, in which the heart rate is measured and the period of the heartbeat is compared to the period of the impedance pneumography signals. When the periods of both signals are essentially identical, i.e., when the impedance pneumography signals are caused by the heartbeat, a carrier threshold value is increased to such an extent that further response to signals caused by heartbeat is prevented. Thus, after this automatic basic adjustment, respiration signals having an amplitude in excess of the amplitude of cardiac artifact signals are automatically detected.
U.S. Pat. No. US-C-3,608,542 discloses a further filtering method for impedance pneumography in which, for suppressing cardiovascular signal components, adjustable narrow-band filters and adjustable level detectors are employed so that good filtering results are provided only if the cardiac artifact signal component to be filtered out is highly periodic.